Thermoelectric dewpoint determining system



J y 0, 1965 I T. N. THIELE 3,195,345

THERMOELECTRIC DEWPOINT DETERMINING SYSTEM Original Filed June 9. 1961 2Sheets-Sheet 1 55 48 50 g 53 52 r 49\ y 80 I l f fill f i a g S 29 I 6770 i 78 72 a 32 73 1 g 58 56 6| F I 6.2 k

INVENTOR. V A TOM N TH/[LE T.,-T T

A TTORNE) July 20, 1965 v 1'. N. THIELE 3,195,345

THERMOELECTRIC DEWPOINT DETERMINING SYSTEM Original Filed June 9. 1961 2Sheets-Sheet 2 BY WWW A Tra/eA/sr United States Patent 0 3 rss 245frrinnMonLEcrnic huwrornr harass untrue svsrnivr Tom N. Thiele,'flcouomowoc, Wis, assignor to Cambridge Systems, Inc., Waltham, Mass, acorporation of Massachusetts Continuation of application Ser. No.1165M), June 9,

1961. This application May It, 1964, Ser. No. 368,446

16 Claims. (6i. 73-17) This invention relates to devices for measuringdewpoint, and more particularly to systems of the thermoelectric coolingtype for determining the temperature at which a vapor begins to condenseon a cold surface.

This is a continuation of application Ser. No. 116,060 filed June 9,1961 by the same inventor and now abandoned.

Present dewpoint measuring devices depend generally upon the supply to apolished surface of a high rate of cooling from either an expendablechemical refrigerant such as Dry Ice, evaporating ether and the like, orfrom a mechanical refrigerating unit of the type utilizing a closedsystem compression and expansion cycle of a suitable refrigerant.

The expendable refrigerant is usually used where the bulk, weight, costand power requirements of a closed system refrigerating unit cannot betolerated and where the data sampling is of relatively brief duration.However, even the use of expendable refrigerants such as Dry Ice andether present severe problems of bulkiness and cumbersome, inefiicientoperation.

These problems are overcome in the present invention of a thermoelectricdewpoint determining system which also incorporates other desirablefeatures and advantages. Among these other desirable features andadvantages are a thermoelectric dewpoint determining system which lendsitself to extremely compact construction and efiicient operation.Another advantage is that of a structure having relatively low powerrequirements for its operation. Further advantages are that it isinherently rugged and requires no mechanical moving parts nor escapinggasses as a cooling source and is relatively inexpensive to manufacture.

From the foregoing it should be apparent that a primary object of thepresent invention is the provision of a dewpoint determining systemwhich is of extremely compact construction.

Further objects are the provision of a thermoelectric dewpointdetermining system which is relatively light in weight, readily handportable and suitable even for lifting to high altitudes by balloonssuch as used in weather studies.

Another object is the provision of a thermoelectric dewpoint determiningsystem which has a relatively low power requirement for its operation.

And a still further object is the provision of a thermoelectric dewpointdetermining system which requires no mechanical moving parts for itsoperation.

Further objects include the provision of a thermoelectric dewpointdetermining system which is rugged, long lived, accurate and relativelyinexpensive to manufacture.

These objects, features and advantages are achieved generally byproviding a thermoelectric refrigerator of the type having a junctionwhich is cooled by the passage of electric current therethrough, amirror surface in heat conducting relation to the junction, and acurrent control circuit arranged for varying current intensity passingthrough the refrigerator in response to changes in reflectivity of themirror surface.

By providing a light sensitive element and a light source positioned forreflection by the mirror of light rays from 3,i%,345 Patented July 20,12365 "ice the light source onto the light sensitive element with thelight sensitive element arranged for controlling current from thecircuit to the refrigerator, an effective arrangement for controllingrate of cooling of the mirror surface as a function of mirrorreflectivity is thereby achieved.

By providing a second mirror surface maintained at ambient temperatureand a second light sensitive element positioned to receive reflectedlight rays from the second mirror surface, an effective arrangement foroperation substantially independently of light source variations andwith reliance on only comparison of reflectivity changes between the twomirrors caused by deposit of condensate on the cold mirror as acontinuous control of the refrigerator current to effectively trackdewpoint is thereby achieved.

By making the light sensitive elements in the form of photoresistivecells incorporated as two legs in a resistance bridge, a very sensitiveand compact arrangement for controlling the electric circuit currentoutput is thereby achieved as well as a desirable arrangement for canceling errors due to change of resistivity of the cells with temperatureand light source variations.

By making the electric circuit in the form of a plurality of directcoupled power transistors, further compactness, low operating powerrequirements, lightness,

reliability and elimination of mechanical working parts are achieved.

By providing a chamber enclosed by opposed walls with a light source andphotoresistive cell on one wall, a light barrier therebetween and a pairof elongated thermoelectric elements extending through the opposite walland electrically joined inside the chamber to form a cold thermoelectricjunction with a mirror in heat conducting engagement with the coldjunction and positioned to reflect light from the light source onto thephotoresistive cell, the other ends of the pair of thermoelectricelements being in heat conducting engagement with a heat exchanger atsubstantially ambient temperature, a reliable and rugged structure forthe formation of condensate on the mirror for tracking dewpoint isthereby achieved.

By providing a second photoresistive cell on the one wall and a secondmirror in the chamber, positioned to reflect light from the source tothe second cell, the second mirror being in heat conductive engagementwith the heat exchanger, and arranging the two photoresistive cells aslegs in the resistance bridge, a practical compact operating controlstructure is thereby achieved.

By making the electric circuit arrangement in the form of an amplifiercontrolled by the resistive bridge in manner to decrease power to thethermoelectric refrigerator with increase in reflected light intensitydifierential at the photoresistive cells, stable automatic operation incoritinuous tracking of the dewpoint is thereby achieved.

By making the thermoelectric refrigerator in the form of a bismuthtelluride PN junction, an efficient thermoelectric cooling arrangementfor wide variations in dewpoint conditions is thereby achieved.

These and other features, objects and advantages of the presentinvention will be better understood from the following description takenin connection with the accompanying drawings of a preferred embodimentof the invention and wherein:

FIG. 1 is a partially block and partially schematic diagram of adewpoint determining system constructed and operating in accordance withthe present invention;

FIG. 2 is a block diagram illustrating a feedback loop applicable to theFIG. 1 embodiment for more clearly showing construction and operationthereof;

FIG. 3 is a plan view of a working embodiment of a portion of theembodiment shown schematically in FIG. 1;

FIG. 4 is a section taken on line 44 of FIG. 3 to more clearly showconstruction;

FIG. 5 is an isometric view of a portion of the embodiment shown inFIGS. 1, 3 and 4 to more clearly show construction.

Referring to FIG. 1 in more detail, a preferred embodiment of thepresent thermoelectric dewpoint determining system has a light sourcewhich includes an electric light bulb 12, preferably of the prefocusedvariety and having a filament 14 connected by lines 13 and 15 to a powersource 16 such as a battery with a switch 18 for opening and closing thecircuit to the filament 14.

When the switch 18 is closed, light rays 20 and 22 from the filament 14are reflected by mirrors 24 and 26 respectively onto photoresistivecells 28 and 3% respectively which are hidden from direct rays of lightfrom the light source 12 by a baffle or barrier 32, The leads 27 and 29of the photoresistive cells 28 and 30 respectively are coupled togetherto a common terminal 31' to form two legs of a resistance bridge 34, theother two legs of which are formed by a potentiometer, resistor .36 oneither side of adjustable contact 38. The adjustable contact 38 iscoupled to the negative terminal of a power source 40 such as a battery,the positive terminal of which is coupled through a switch 41 to thecommon terminal 31 of the photoresistive cells 28 and 30.

The other lead 43 of the photoresistive cell 30 is connected at terminal42 through an adjustable contact 4-4 to a potentiometer resistor 46which with a series switch 47 is connected across a power source 48 suchas a battery. The negative terminal of the battery 48 is connectedthrough an electric cable 50 to the collectors of transistors 68 and 70in a high gain transistor amplifier fitland to the negative terminal ofa battery 49, the positive terminal of which is connected to thecollector of a third transistor 72 in the transistor amplifier 60 and toa terminal 51. The terminal 51 is connected through a fuse 53, a switchand an electric cable 100 to one leg 57 of a thermoelectric refrigerator52 utilizing Peltier eifect, and sometimes referred to as Peltier typecooler, for cooling the mirror 26 as shown and described in greaterdetail in connection with FIGS. 4 and 5. Another leg 59 of thethermoelectric refrigerator 52 is connected through an electric cable 54to the collector 56 of a power output.

transistor 58 in the high gain transistor amplifier 60. The power outputtransistor 58 has a base 61 coupled to the emitterof the transistor 72and has an emitter 62 coupled through an electric cable 64 and resistor66 to a terminal 65 to which is connected by a lead 77 of thephotoresistive .cell 28 and one end of the potentiometer resistor 36 inthe resistance bridge 34.

In the high gain transistor amplifier 60, the transistors 68, 70, 72 and58 are direct coupled. The base 71 of transistor 72 is connected to theemitter 73 of the transistor 70 and through a resistor 74 to theelectric cable 64, while the base 69 of the transistor 70 is connectedto emitter 67 of the transistor 68 and through a resistor 76 to theelectric cable 64 to enhance stability of operation of the amplifier 60.The first transistor stage 68 has a base 63 connected to the bridgeterminal 65 side of the resistor 66. A direct current power source suchas a low voltage, high ampere capacity battery '75 has its negativeterminal connected to the terminal 51 and its positive terminal to theemitter 62 of the output transistor 58.

A blower 78 which may be of conventional design is arranged to direct bya nozzle or tube 79 a steady stream of air or other gas whose dewpointis to be determined onto the cold mirror 26. Also, a thermocouple 80 onthe face of the cold mirror 26 is connected by electric cables 82 to asuitable meter 84 such as a galvanometer calibrated to give thetemperature at the thermocouple 80. Where the accuracy of a thermocoupleis not necessary, a thermistor may be used in place of the thermocouple80 and an ammeter in place of the galvanometer 84 for further economyand compactness of the. system.

.cool mirror 26. When the bridge 34 is in this balanced condition, theadjustable contact 44 is set on the potentiometer resistor 46 to supplyafull bias current for driving the amplifier 66 to maximum output,thereby feeding current to the thermoelectric cooler or refrigerator 52for maximum cooling rate of the mirror 26. As the mirror 26 drops intemperature to the dewpoint, a condensation deposit forms thereon fromthe vapor in the gas or air stream emanating from the tube or nozzle 79of the blower 78. The intensisty of light rays 22 reflected from thecold mirror 26 onto the photoresistive cell 30 is thereby diminished andcauses an unbalance in the resistance bridge 34 which, being in serieswith the bias current through the contact 44, reduces the input bias tothe amplifier 66. This cause a corresponding reduction in the amplifieroutput current to the thermoelectric refrigerator 52 with acorresponding reduction in rate of cooling until equilibrium is reached.The accuracy with which this equilibrium condition is maintained andreflects the true dewpoint temperature may be calculated in thefollowing manner:

Referring to the block diagram of the feedback loop in FIG. 2, thefollowing can be written by inspection:

From which it is found that:

I .=T k /(1+Gk k and the error To-T at balance becomes:

'T=temperature of the cold mirror 26 in C. (degrees The above analysisassumes that the thermoelectric refrigerator 52 has zero time constantand that operation is in the linear range where AT AT Typical values ofthese constants are found to be T =20 C.,

. k =l0 k =2, andG=5 10 and have been found to give an error of lessthan 02 C. at a dew point of 20 C. below ambient, the error beingproportionately less for smaller temperature differences. These typicalvalues are cited here for illustration only and are not intended aslimitations since they may be changed by selection of components withother parameters to meet desired conditions of use of the presentinvention.

Since the control signal current k (T T) can decrease only whencondensate is formed on the cold mirror 26, at all times when nocondensate exists, there will be a constant maximum current flowing. Theminimum gain required in amplifier 60 is that amount just sufficient toprovide rated l to the refrigerator with the maximum control signalcurrent input to the amplifier 69. Since this leaves no safety margin inthe event of loss of gain, a value several times this minimum should'beused. By way of rent, a minimum gain required in the amplifier 60 wouldbe 1.6x in which case to insure a proper safety factor and betterequilibrium accuracies an actual gain of 5 10 in amplifier 60 should beprovided. Such gain and current characteristics may be achieved by usingthe following exemplary parameter values for components in the FIG. 1embodiment:

Photoresistors 28 and 30 50,000 ohms each. Potentiometer 36 100,000ohms. Potentiometer 46 75,000 ohms. Resistor 66 500,000 ohms. Resistor74 25,000 ohms. Resistor 76 100,000 ohms. Transistor 58 Type 21 1677Bendix. Transistors 68 and 7h Type 2N112 Raytheon. Transistor 72 Type2N1138 Bendix. Batteries 16 and d9 1.5 volts.

Battery 40 67.5 volts.

Battery 48 4 volts.

Battery 75 2 volts.

Bulb 12 Type 222 prefocused.

In practice it has been found that throughout the operating range,changes in gain in the transistors introduce only very small error inindicated dewpoint since the circuit will then come to equilibrium witha slightly heavier layer of dew. The layer of dew at equilibrium is sothin, however, that a considerable margin exists before a significanterror occurs in the temperature of the mirror 26.

Referring to FIG. 5 in more detail, the thermoelectric refrigerator 52has preferably a pair of semi-conductor elements or legs 57 and 59 of asolid state material such as bismuth telluride. The leg 57 is of P typesemi-conductor material (that is, having a predominance of acceptor typeatoms or holes) and the leg 59 is of N type semi-conductor material(that is, having a predominance of donor type or carrier atoms). Thelegs 57 and 59 are each soldered at one of their respective ends to heatsink blocks 86 and 88 respectively of a highly heat conductive and lowelectrically resistive material as copper to form the hot junctions forthe thermoelectric refrigerator 52.

For efficient operation of the thermoelectric refrigerator 52, it isnecessary that very low contact resistance exist at the junctionsbetween the legs 5'7 and 59 and the heat sink blocks 86 and $8respectively. Such low contact resistance may be achieved by carefullyfollowing a procedure which includes sanding the surfaces to be joined,after which they are washed, followed by a light sandblasting and againwashed, first with ethanol to remove any grease and then with distilledwater. Thereafter, a light plating of nickel is applied, the platingcontinued until a matte finish is obtained. The surface is then tinnedwith a low melting point solder. The copper heat sink blocks 86 and 88are also given a plating of nickel and similarly tinned. They are thenheated to the solder melting point, the legs 5'7 and 59 positioned inplace on the blocks 86 and 38 respectively, and the assembly cooled.This procedure is capable of producing junctions with electrical contactresistances of only 10 micro-ohms or less, which is very important forefiicient operation of this type of cooler.

The mirror 26 which is preferably of metallic heat and electricallyconductive material is also soldered in place in similar manner at theother ends of the legs 57 and 59 to form a cold junction to provide api-coniiguration thermoelectric refrigerator 52.

The legs 57 and 59 are of a length 90 such that they are about 3.5 timestheir individual cross sectional area provide better understanding ofthe present invention and are not intended as a limitation thereof inview of their being variable by component parameters of the device.

The copper block heat sinks 86 and 83 are fastened, as by screws 94,pressurably against so as to be in heat conductive engagement with aconventional heat exchanger 92 of such highly heat conductive materialas aluminum or copper and having cooling fins 93 for increased heatdissipation. A very thin membrane layer or sheet 96 of such electricallyinsulating material as mica is placed between the copper heat sinks 86and 88 and the heat exchanger 92 to provide electrical insulation fromthe heat exchanger 92 with a minimum of heat conduction interferencebetween the blocks 86 and 88 and the heat exchanger 92. Fastening of theblocks 86 and 88 to the heat exchanger 92 may be by screws 94electrically insulated from the heat exchanger 92 by dielectric flangedcollars 93 (FIG. 4).

A screw 94 on the heat sink 88 may be used as a terminal for theelectric cable 100 for coupling to the negative terminal of the battery75. A screw 94 on heat sink 36 may be used as a terminal for theelectric cable 54 for coupling to the collector 56 of the outputtransistor 58 for operation of the thermoelectric refrigerator 52 inconjunction with the amplifier 60 as described above.

The mirror 24, which may be similar to the mirror 26, is fixed in heatconductive engagement to one end of a highly heat conductive rod 102,such as of aluminum or copper, the other end of which is fixed in heatconductive engagement with the heat sink 86 in any suitable manner suchas by soldering or other mechanical fastening means. The mirror 24- isthereby maintained at substantially the temperature of the heat sink 86which is close to ambient.

Referring in more detail to FIGS. 3 and 4 wherein are shown views of apreferred working embodiment of the major components illustratedschematically in FIG. 1 and where for clarity of illustration, thewiring and some of the elements shown in FIG. 1 have been omitted. Inthe practical embodiment in FIGS. 3 and 4, a mounting block or basemember 104 of any suitable material as wood, plastic or metal has fixedthereto, as by screws 108, an angular support member 106. The supportmember 106 has fastened to it by screws 110 the heat exchanger 92 withthe semi-conductor legs 57 and 59 and the rod 102 extending horizontallythrough openings 112 and 113 in the support member 106 so that themirrors 26 and 24 are in a chamber 114, one wall of which is formed bythe angular support number 106 and an opposite wall of which is formedby another angular support member 116 fixed to the base member 104 byscrews 118. The angular support member 116 carries therein a threadedsocket 120 for fixing the bulb 12 in position such that light rays 20and 22 emanating from the filament 14 are reflected from the mirrors 24and 26 onto the photoresistive elements 28 and 30 respectively which areheld in place in recesses in the light barrier and holder member 32which also serves to prevent light at the filament 14 from directlyreaching the photo resistive cells 28 and 30. A similar opaque barrier122 at the mirrors 24 and 26 helps prevent scatter light from reachingthe mirrors 24 and 26. The light barrier 32 also carries an opening 12 4directed toward the cold mirror 26 and has therein the tube or nozzle 79of the blower '78 for providing a steady stream of air or other gasunder test onto the cold mirror 26.

The chamber 114 is further enclosed by oppositely disposed side walls126 and 128 and top and bottom walls 130 and 132, all of any suitableopaque material to provide a complete closure which excludes light andair or gas other than that from the bulb 12 and nozzle 79.

A frame or housing 134 is fixed by spacer rods 136 to the angularsupport member 116 and is arranged to conveniently carry such of theFIG. 1 embodiment components as batteries 48 and 16 and potentiometers34 and 36.

A third angular support member 137 is also fastened thereto by screws142 a second heat exchanger 140 which may be similar to the heatexchanger 92 for carrying in heat. conductive engagement therewith inconventional manner, the power transistors such as transistor 58 formaintaining it at substantially ambient temperature during operation.The practical working embodiment shown in FIGS. v3 and 4 When combinedwith the circuitry and the remaining battery supplies shown in FIG. 1will operate to provide continuous dewpoint temperature tracking asdescribed above.

This invention is not limited to the specific details of constructionand operation herein described as equivalents will suggest themeselvesto those skilled in the art.

What is claimed is:

1. In combination, a pair of light sensitive members, a light source, apair of mirrors, each of the mirrors arranged to reflect light from thesource onto a respective one of the light sensitive members, athermoelectric cooler having a Peltier eflect type cold junction incooling relation to one of the mirrors and with a cooling rate.dependent on intensity ofcurrent flow in a cooling direction through thePeltier effect type junction, and an electric circuit means coupled tothe cooler and the light sensitive members for varying said intensity ofelectric current flow in the cooling direction through said Peltiereflect type junction in response to change in reflected light on saidlight sensitive members.

2. The combination asin claim 1 wherein the light sensitive members arephotoresistive cells.

3. The combination as in claim 1 wherein the electric circuit meansincludes aresistance bridge and the light sensitive members arephotoresistive cells in the resistance.

bridge and arranged to place the bridge in responsive relation todifferences in reflected light intensity from the mirrors and in controlrelation to the electric circuit means.

4. The combination as in claim ,1 wherein the thermo- 7 electric cooleris a bismuth telluride PN junction.

5. In a dewpoint determining device of the type having a light sourceand a pair of mirrorsfor reflecting light from the source, thecombination of electronic amplifier means having a variableunidirectional current output, a

' Peltier type cooler in cooling relation to one of the mir- I spect tothe reflected light intensity from said othermirror 6. In combination, aheat exchanger, a pair of elongated PN solid state members with eachhaving two ends, one of the ends of each being electrically insulatedfrom and coupled in heat conducting relation to the heat exchanger andthe other end of each being electrically coupled to.

form a PN junction for electric current conduction therethrough, mirrormeans in heat conducting engagement with the electrically coupled ends,a pair of photoresistive cells, a light source positioned for causingdirect reflection of light by the mirror means onto one of thephotoresistive cells and scatter light from the mirror means onto theother photoresistive cell, electric current means in responsive relationto the photoresistive cells and in current conducting relation to thepair of solid state mem hers.

7. In combination, a thermoelectric refrigerator of the type having ajunction Which is cooled by the passage.

of electric current in a cooling direction therethrough, a polishedsurface in heat conducting relation to the junction, current controlmeans arranged for changing intensity of current passing through therefrigerator junction in said cooling direction in response to changesin reflectivity of the polished surface, and a transistor amplifier inresponsive relation to the control means and in current conductingrelation to the junction for supplying said cooling current through thejunction. 7

8. In combination, a light sensitive member, a light source, a mirror,the mirror arranged to reflect light from the source onto the lightsensitive member, a thermoelec tric cooler having 0 Peltier eifect typecold junction in cooling relation to the mirror and with a cooling ratedependent on intensity of current flow in a cooling direction throughthe Peltier-effect type junction, and electric circuit means coupled tothe cooler and light sensitive member for varying said intensity or".cooling direction electric current flow through said Peltier effect typejunction in relation to variation in reflected light on said lightsensitive member.

9.'In combination, a thermoelectric cooler having a Peltier effect typecold junction with a cooling rate which varies with the rate of electriccurrent flow in a cooling direction through the Peltier effect type coldjunction, a highly reflective surface in heat conductive relation to thecold junction for receiving dewpoint condensate deposit, electriccircuit meansfor supplying operating current flow in the coolingdirection through the Peltier effect type cold junction of thethermoelectric cooler, and light sensitive means in responsive relationto said surface and in control relation to the circuit for limiting theintensity of said current flow in'the cooling direction through saidPeltier effect type junction in response to depth of said condensatedeposit on the highly reflective surface.

10. In combination, a thermoelectric cooler having a Peltier effect typecold junction with a cooling rate which varies with the intensity offlow of operating current in a cooling direction through said Peltiereflect type junction, condensate. sensitive means in heat conductiverelation to the Peltier effect type cold junction, electric circuitmeans for supplying operating current in the cooling direction throughthe Peltier effect type cold junction of the thermoelectriccooler, andmeans in responsive relation to the condensate sensitive meansand incontrol relation to the circuit means for varying the intensity of flowof said operating current in the cooling direction through the Peltiereflecttty-pe junction in inverse relation to variation in amount ofcondensate at said. condensate sensitive means.

11. In a device for determining the dewpoint of a gas, the combinationof a thermoelectric cooler having a hot and a cold junction whose ratesof cooling and heating respectively vary with the intensity of flow ofoperating current in a single direction through said junctions, heatexchanger means coupled to the hot junction for maintaining said hotjunction at substantially the temperature of said gas, electric circuitmeans coupled to the thermoelect-ric cooler for supplying said singledirection operating current through said junctions, and condensateresponsive meansin heat conductive engagement with the cold junction andin control relation to said circuit means for varying the intensity ofsaid operating currentin inverse relation to amount of said condensateon said cold junction.

12. The method of tracking the dewpoin-t of a gas comprising the stepsof passing an electric current through a thermoelectric cooler in adirection to cause a Peltier effect type cooling of a surface forreceiving a dew layer, passing the gas whose dewpoint is to be trackedover the surface being cooled to cause condensate deposit on saidsurface for creating said dew layer, varying the intensity of saidelectric current in said cooling direction through saidthermoelectr-iccooler in inverse relation to the thickness of said dew layer toestablish an equilibrium condi tion characterized by the maintenance ofa substantially constant dewlayer of condensate from said gas on saidcooled surface, and measuring the temperature of said surfiace duringsaid equilibrium condition.

13. In combination, a heat exchanger having a metallic heat conductingbase, a thin dielectric membrane on said base, a pair of copper heatsink blocks on the membrane, a P type elongated solid state member andan N type elongated solid state member, each of the solid state membershaving two ends, one of the ends of the P type member fixed in heat andelectrically conductive relation to one of the copper blocks, one of theends of the N type member fixed in heat and electrically conductiverelation to the other copper block, a cur-rent conductive electricaljunction coupling the other ends of the P and N type members, a mirrorfixed in heat conducting relation to the junction, current meanselectrically coupled across the blocks for passing current through thejunction and P and N members in a direction causing thermoelectriccooling at the junction, and means in responsive relation to the mirrorfor determining the temperature at which the reflective capacity of themirror changes.

14. In combination, a housing having opposed walls inclosing a chamber,an electric light bulb fixed substantially centrally of one of saidopposed walls in the chamber, two photoresistive cells, one cell fixedat one side of the light bulb and the other cell fixed at the other sideof the light bulb at the one wall in the chamber, a barrier in positionfor obstructing passage of light rays in a direct line from the bulb tothe cells, an elongated thermoelectric refrigerator having cold and hotjunctions with the cold junction extending through the opposite wall toa position in the chamber distal from the light bulb and one of thecells, a mirror in heat conducting engagement With the cold junction andpositioned to reflect light rays from the light bulb onto said one cell,a heat exchanger outside the chamber and in heat conducting engagementwith the hot junction, a second mirror in the chamber positioned toreflect light rays from the light bulb onto the other photoresistivecell, a heat conductive member in heat conducting engagement With thesecond mirror and heat exchanger, electric current means coupled to thecells and the thermoelectric refrigerator, and means for blowing astream of gas under test onto the first mentioned mirror.

15. In combination, a thermoelectric refrigerator of the type having ajunction with a reflective surface which is cooled -by the passage ofelectric current in a cooling direction through said refrigerator,direct current means for supplying said current through saidrefrigerator, and current control means in responsive relation to saidreflective surface and in control relation to said current means forchanging the intensity of said cooling direction current passing throughsaid refrigerator in response to changes in reflectivity of thereflective surface.

16. The method of detecting the dewpoint of a gas comprising the stepsof passing the gas over a reflective surface, Peltier effect cooling ofthe reflective surface in proportional relation to rate of an electriccooling direction current causing said Peltier effect cooling,decreasing rate of the electric cooling current and thereby said rate ofcooling of the reflective surface in proportional relation to decreasein reflectivity of the reflective surface from dew formation on thecooled reflective surface until an equilibrium condition is reached.

References Cited by the Examiner UNITED STATES PATENTS 2,624,195 1/53Van Alen 73-17 2,671,334 3/54 Gunn 73---17 2,979,950 4/61 Leone 73-17 XRICHARD C. QUEISSER, Primary Examiner.

Disclaimer 3,195,345.T0m N. Tllielc, ()conomowoc, \Vis. THERMOELECTRICDEV- POINT DETERMINING SYSTEM. Patent dated July 20, 1965. Disclaimerfiled Feb. 11, 1976, by assignee, [JG (E G, Inc. Hereby enters thisdisclaimer to claims 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 15 and 16of said patent.

[Oflic'z'al Gazette May 25, 1.976.]

16. THE METHOD OF DETECTING THE DEWPOINT OF A GAS COMPRISING THE STEPSOF PASSING THE GAS OVER A REFLECTIVE SURFACE, PELTIER EFFECT COOLING OFTHE REFLECTIVE SURFACE IN PROPORTIONAL RELATION TO RATE OF AN ELECTRICCOOLING DIRECTION CURRENT CAUSING SAID PELTIER EFFECT COOLING,DECREASING RATE OF THE ELECTRIC COOLING CURRENT AND THEREBY SAID RATE OFCOOLING OF THE REFLECTIVE SURFACE IN PROPORTIONAL RELATION TO DECREASEIN REFLECTIVITY OF THE REFLECTIVE SURFACE FROM DEW FORMATION ON THECOOLED REFLECTIVE SURFACE UNTIL AN EQUILIBRIUM CONDITION IS REACHED.